This invention relates to a chill mold consisting of a steel cylinder for making piston castings, especially from aluminum alloys, preferably for internal combustion engines, and having an inserted internal core reproducing the internal contours of the piston.
The chill casting process is assuming importance in the production of pistons from aluminum alloys. In this process the metal is cast in reusable metal molds, the molds being filled generally either by gravity or by low gas pressure. The chill casting process is well suited to the production of pistons from aluminum alloys because aluminum alloys have good castability and the casting temperatures assure that the mold can be used for a relatively long period of time -- for approximately 30,000 to 50,000 castings -- and thus assure good economy. Low and medium numbers of piston castings are generally made in multisectional hinged molds operated by hand, into which a multi-sectional internal core of metal, especially steel, assembled by hand, is inserted. Usually in this type of chill the piston head is facing downwardly and the pouring gate discharges into one of the two laterally mounted feeders.
For larger quantities of piston castings the use of semiautomatic casting machines is warranted, which, like the manually assembled chill, are fed with molten metal by means of a dipping ladle. The piston head in this type of casting is facing upwardly since the multi-sectional core has to be removed downwardly. Aside from pouring in the molten metal such casting machines need only the actuation of a control button in order to bring about automatically the pneumatic or hydraulic closing of the multi-sectional chill, followed by the reopening of the chill after a predetermined time and the removal of the core sections, and then the removal of the piston casting. Whereas in these above-described chill casting processes the filling of the chill is performed by gravity and the solidification takes place under air pressure, in the low-pressure casting process the metal is forced into the chill by a gas pressure of about 0.2 to 0.3 atmospheres gauge, and it hardens in the chill under this pressure. In this process the bottom aperture of the chill is located on a riser tube from a reheating furnace which contains the molten metal and is hermetically sealed with a cover. The chill, in which the piston head faces downwardly, is filled with molten metal through the riser tube. The flow of the molten metal into the chill may be controlled through the gas pressure and the inflow cross-section, so that the cavity in the chill can be filled without turbulence. Whereas small pistons are manufactured by the above-described methods, the casting of large pistons requires particularly great experience. In particular, the treatment of the melt and the foam-free injection of the metal into the chill require special measures. The large piston chills and multi-sectional steel cores, which are very heavy in comparison to chills for small pistons, are brought up to the casting furnace with cranes as a rule, and are filled directly from the tilting furnace.
The so-called fine-grain method of casting large pistons offers a substantial improvement of technological properties in comparison with the last-described method. In this process a steel cylinder having a water-cooled bottom is filled with the molten piston alloy in which a special sand core is then suspended in a precisely centered manner. Gas burners directed against the outer periphery of the steel cylinder serve to keep the melt hot and prevent the upwardly pointing thin-walled cross-sections of the piston from hardening more rapidly than the piston head. The steel cylinder is then lowered into a water bath in a timed manner causing the solidification to progress strictly from the bottom upwardly at a prescribed rate of speed. In this manner a controlled fine-grain solidification of the material is achieved, thereby creating good technological properties.
Excellent technological properties are expected not just in the case of large pistons. In fact, progress in the development of internal combustion engines towards ever increasing power output is imposing more stringent requirements on the quality of small pistons.